There are three types of chemical sensors: chemical sensors used in conductometry, chemical sensors used in amperometry and chemical sensors used in potentiometry. For example: pH electrodes, ion-selective electrodes, ISFET (ion-selective field effect transistors), enzyme electrodes, biosensors, etc. are widely used in chemical, biochemical, biotechnological, environmental protection and medical analyzers such as a: pH-meter, ion analyzer, polarography, chemical analyzer, bioanalyzer, bioreactor, ion chromatography, flow-injection analyzer, etc. Moreover, Chemical sensors are used in quality control analysis, on-line analysis and monitor-control apparatus for chemical manufacturing processes. U.S. Pat. No. 4,897,128 discloses a method of controlling the ionic concentrations of reactants in a zinc phosphate coating sink by using pH-electrodes and fluorine-ion selective electrodes.
A chemical sensor can convert a specific chemical signal (i.e. concentration of a certain component of a sample) or the sum of many chemical signals into an electronic signal such as electric potential, resistance, or current. However, in order for the users to understand the physical meanings of a chemical signal, this electronic signal still needs to be further processed, stored, and/or displayed by an signal processing equipment. For example, a pH-electrode has to be incorporated with a pH meter to determine the pH value of a solution. Similar requirement applies to the usage of a chemical analyzer, and pH monitor-control equipment. A chemical sensor for amperometry can determine the reaction current in the potential static condition by relying on a potentiostat, and then indirectly obtain the concentration of a specific specie. Similar requirement applies to the usage of a chemical analyzer and a biochemical analyzer. A chemical sensor for conductometry can determine the conductivity by relying on a conductometer, and the determined conductivity can then be used to indicate the ending of a conductometric titration, or used as the standards of ionic concentrations in ion chromatography. Each of these kinds of signal processing equipment has a specific usage, and cannot be exchanged for use in another chemical sensor. For example, a pH-meter can not be without a conductometer or a coulometer, and it also can not be extended to another use without a special design, a potentiometer cannot be extended to be used in potentiostatic coulometry.
Generally speaking, conventional signal processing equipment can be divided into 4 categories. The first category includes the simple instruments which cannot be connected to a computer or a recorder. For example, pH-meters (types 704, 620, 588) of Metrohm, Switzerland, do not have very powerful functions, and do not have the ability to execute data communication with other instruments like chemical analyzers.
The second type of signal processing equipment, however cannot be connected with a computer externally but, is able to communicate with an external recorder through its analog signal output node, and, therefore, enhance its function. Examples are the pH-meter (PHM82) of Radiometer, Denmark and the, Potentio/Galvanostat and Coulomb/Amperohour Meter of Nichia, Japan. The functions of these signal processing equipment are still limited. Although an analog signal output node is available, it is still physically difficult to execute data communication with other instruments.
The third type of signal processing equipment cannot be externally connected with either a computer or a recorder but, has its own built-in display and printer. Examples are the modular biological fluid analyzer disclosed in U.S. Pat. No. Des. 330,770, and a clinical chemistry analyzer disclosed in U.S. Pat. No. Des. 332,314. These built-in functions clearly can not be compared with those of a computer. For example, the resolution of a computer monitor is better than a built-in display of a signal processing equipment. A computer also has superior data processing/storing capabilities and various accessories which can be mounted into the computer easily. Moreover, the analyses and data processing functions of this type of signal processing instruments cannot be extended or enhanced.
The fourth type of signal processing equipment can be connected with a computer externally in order to enhance its data processing/storing/display ability. Example include the voltammetry Model 693 VA Processor from Metrohm, Switzerland; the PHM 85 pH-meter from Radiometer, Denmark; the Potentiostat/Galvanostat Model 273A from EG&G, U.S.A.; a chemical analyzer disclosed in U.S. Pat. No. 4,935,875; and the on-line biological inhibition/toxicity detector disclosed in U.S. Pat. No. 5,106,511. This type of signal processing equipment contains a central processing unit. For example, line 17, column 5 of U.S. Pat. No. 5,106,511 and line 45, column 6 of U.S. Pat. No. 4,935,875 state that these signal processing equipment use a Model 6809 microprocessor (Motorola, U.S.A.), ROM, RAM, timer, display or monitor, keyboard or I/O port, ADC, etc. (For further details, please refer to FIG. 1 of U.S. Pat. No. 4,935,875 and its explanation.) In addition, when these signal processing equipment are to be connected externally with computers, RS-232 or GPIB cards needs to be Used as the medium for data communication.
In order to extend the analysis and data processing functions, the inventors have focused their research on the structure of the fourth type of signal processing equipment. The result is that except for some minor components such as ADC, the primary components such as CPU, ROM, RAM, timer, monitor, keyboard, I/O port, printer and disk drive, are all included in a computer. This is advantageous because the primary components of a computer are generally more powerful and more compatible to external accessories than the built-in components in the fourth type of signal processing equipment. Therefore, the fourth type of signal processing equipment may essentially be replaced by a computer. In addition, the minor-components such as ADC can be easily purchased in the market. Accordingly, it is possible to use an ADC bought from the market to directly convert the analog signals from a chemical sensor into digital signals, and transfer the digital signals to a computer where they are processed. If this can be accomplished, the signal processing equipment used at the present time can be entirely replaced by a computer with modifications. Nowadays, some mechanical sensors or thermal sensors are using the same idea of replacing signal processing equipment with computers and ADCs. However, this idea has not been used in chemical sensors.
Based on the above analyses, the inventors used a market-purchased ADC to connect a chemical sensor (i.e. a pH electrode) and a computer. In other words, the output signals from a chemical sensor were received in a series as follows: "chemical sensor.fwdarw.ADC.fwdarw.computer". However, the results showed that although a large number of data were collected, the average value of these data could not represent the actual value accurately because the average values were not consistent for several runs repeated by the same procedures. The deviations were large and no pattern could be found.
After more intensive research, the inventors found out that the addition of a voltage follower could solve the existing problem. That is to say, if the connection is in a series of "chemical sensor.fwdarw.voltage follower.fwdarw.ADC.fwdarw.computer", the output signals of a chemical sensor can be easily and accurately obtained.
Furthermore, current ADCs in the market often have an additional function of Digital-to-Analog Conversion (DAC) at the same time. Therefore, it is theoretically possible to use a DAC to convert the digital signals sent by a computer into analog signals, and therefore use a chemical sensor to execute voltammetry applications; or under potentiostat conditions, to execute amperometry and obtain a concentration of a certain component of a sample. The actual experimental results showed that although the voltage output of the DAC was stationary, the electric potential of the working electrode was fluctuating. However, this problem can be solved by the addition of a potentiostat circuit. Similarly, a potentiostat circuit can be used to solve the same problem in potentiometry under galvanostat conditions.
In the conventional signal processing equipment of a chemical sensor for conductometry, a transducer has to be added to reduce the voltage of an alternating current source for the conductance cell. However, the inventors found that by executing a control program in the computer, the DAC can be used as an alternating current source.
Based on these three discoveries, a system containing a computer, an ADC/DAC, a potentiostat circuit, a voltage follower and a proper computer program which can be executed in said computer can be used to carry out amperometry, potentiometry, conductometry, and voltammetry for different chemical sensors. In other words the invention provides a method and system having the combined functions of a potentiometer, a pH meter, an amperometer, a conductometer, a potentiostat, a Galvanostat, a voltammetric processor, and thereby substantially covers all the equipment which use chemical sensors or any extension uses of these equipment, e.g. as a conductometer used in ion-chromatography. In contrary, the conventional signal processing equipment for chemical sensors have their own specific usages that cannot be exchanged. For example, a pH meter can be used only as a pH meter, not a potentiostat, and a voltammetric processor can not be used as a conductometer at the same time.
In addition, because ADC/DAC cards on the market usually have DIO (digital input/output) functions, and hence they can also be used as the control of a pump or a valve, the system described above can generally be connected with other accessories (if necessary) to be used as a chemical analyzer, bio-chemical analyzer, clinic analyzer, pH/electric potential/conductance automatic titration meter, ion chromatography, polarography, and a quality control, on-line analysis and monitor-control equipment of a chemical manufacturing process.